Method of transposing sheet conductors

ABSTRACT

Methods of radially transposing radially adjacent electrically conductive sheet materials in an electrical winding, which include forming notches in opposite edges of the conductors to be transposed and directing each conductor through the notch in the other. The notches are formed, while increasing the crosssectional area of the conductor adjacent the notch, by folding back a section of conductor to form a notch, and electrically joining certain of the edges of the folded section to the surface of the adjacent sheet material. The transposition is performed by directing each sheet through the notch in the other, and changing the relative positions of the supply roll of sheet material.

United States Patent Inventors Frank W. Benke Sharon, Pa.; Gary E. Uitto, Niles, Ohio App]. No. 75,127

Filed Sept. 24, 1970 Patented Jan. 11, 1972 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

Original application July 5, 1968, Ser. No. 742,723, now Patent No. 3,587,169. Divided and this application Sept. 24, 1970, Ser. No. 75,127

METHOD OF TRANSPOSING SHEET 605,624; 113/1 16 R, 116W, ll6Y;310/213; 174/33, 34; 72/379; 336/187, 223

Primary Examiner-John F. Campbell Assistant Examiner-Carl E. Hall Attorneys-A. T. Stratton, F. E. Browder and D. R. Lackey ABSTRACT: Methods of radially transposing radially adjacent electrically conductive sheet materials in an electrical winding, which include forming notches in opposite edges of the conductors to be transposed and directing each conductor through the notch in the other. The notches are formed, while increasing the cross-sectional area of the conductor adjacent the notch, by folding back a section of conductor to form a notch, and electrically joining certain of the edges of the folded section to the surface of the adjacent sheet material. The transposition is performed by directing each sheet through the notch in the other, and changing the relative positions of the supply roll of sheet material.

METHOD OF TRANSPOSING SHEET CONDUCTORS CROSS-REFERENCE TO RELATED APPLICATION This application is a division of application, Ser. No. 742,723 filed July 5, 1968, now US. Pat. No. 3,587,169 which is assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates in general to methods of transposing electrically conductive sheet materials in windings for electrical inductive apparatus, such as transformers and reactors, and more particularly to methods of transposing sheet conductors in such windings.

2. Description of the Prior Art Constructing windings for electrical transformers of metallic foil, sheet or strip materials, has certain advantages over windings constructed of wire-type conductor. For example, windings constructed of wire or strap conductor require electrical insulation between radial layers of conductors, as well as turn-to-turn insulation, which increases the size of the winding. Further, with wire or strap wound windings, it is difficult to accurately determine the electrical center of the winding, i.e., the axial location which divides the ampere turns equally, especially when the winding is tapped. When the primary and secondary windings of the transformer are assembled in concentric adjacent relation, any axial displacement of their electrical centers creates a force during short-circuit stresses which forces the windings axially apart.

Electrical windings constructed of sheet material, such as copper or aluminum strip or foil, wherein the winding includes a plurality of superposed conductor turns, requires only turnto-turn insulation, as the conductor turns extend across the axial dimension of the coil; or where axially aligned electrically connected part-coils are used, the conductor of each art-coil would extend across the axial dimension of its associated part-coil. Since the turn-to-turn voltages are relatively low, the turn-to-turn insulation may be a thin strip of insulating material, such as kraft paper, which is wound between the conductor turns at the time the winding is constructed, or it may be a thin coating of an insulating enamel applied to one or both of the major surfaces of the sheet material. Thus, the space factor of an electrical winding formed of electrically conductive sheet materials is excellent, resulting in a smaller winding, which reduces the size of the magnetic core and the casing, and thus reduces the size, weight and cost of the transformer. Also, since the electrical center of a winding constructed of sheet material is the same as its mechanical center, there will be less force tending to separate concentric windings axially during short-circuit stresses. The sheet winding is also inherently stronger than a winding formed of wire or strap conductor, as the individual conductors of a wire-type winding may be deformed by short-circuit stresses, eventually causing turn-to-turn and layer-to-layer short-circuits, while the sheet conductor turns resist deformation, and are not as subject to movement and vibration which mayabrade and wear the insulation disposed between two conductors,

When the current rating of an electrical winding is increased, more conductor cross-sectional area is required to carry the increased current, if the winding temperature is to be maintained below the maximum temperature allowable for the class of insulation used. However, since eddy current losses are proportional to the square of the dimension of the conductor at right angles to the leakage flux, it is common to subdivide the required conductor area into two or more parallel connected conductive elements, which are electrically insulated from one another, except at their common connections at the start and finish of the winding, and at any tap connections to the winding. The stranding of the conductor is easy to accomplish with wire, strap, or sheet type conductive materit. al. With wire or strap conductors, the required plurality of strands are electrically insulated from one another, and they may be taped together to form a single structure which facilitates winding, With sheet, the required number of sheet conductors are wound simultaneously, in superposed relation, with electrical insulating means disposed between the conductors.

Subdividing the conductive cross-sectional area required into a plurality of parallel connected elements, however, introduces losses due to circulating currents in the parallel connected elements. Losses due to circulating currents may be reduced by transposing the relative radial positions of the conductive elements, in an effort to subject each element to the same net leakage flux and, therefore, equalize the induced voltages in the various elements and provide a net induced around each loop of substantially zero. When using wire or strap conductor, the transposing of the individual strands is relatively easy, with a number of transposing arrangements being known in the art for transposing any number of parallel connected strands.

Transposing the relative radial positions of a plurality of sheet conductors, which are wound about a winding form or mandrel, each from a separate roll or supply of sheet material, and with the conductor turns all being in superposed or nested relation, presents a much more difficult problem. The transposing arrangement, and methods of accomplishing the transposition, should perform the necessary radial interchange of sheet conductors without being unduly complicated, without substantially reducing the effective cross-sectional area of the conductor, and without substantially increasing the size of the winding.

SUMMARY OF THE INVENTION Briefly, the present invention discloses new and improved methods for radially transposing electrically conductive sheet materials. One of the disclosed methods of radially transposing first and second electrically conductive sheet materials in an electrical winding comprises the steps of forming notches in opposite edges of first and second sheet materials, by making two spaced cuts in the edge of each of the sheet materials in which the notch is to be formed, folding back the section of sheet material between the spaced cuts, in each of the sheet materials, and electrically joining the cut edges of the folded sections to the immediately adjacent portion of the sheet material, in each of the sheet materials. The sheet materials are transposed by directing each through the notch in the other.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a perspective view of an electrical winding formed of first and second electrically conductive sheet materials, which may utilize the teachings of the invention to transpose the radial positions of the sheet materials;

FIGS. 2A through 2D are schematic diagrams of typical transposing arrangements which may utilize the methods of the invention;

FIGS. 3A and 3B illustrate the steps in a method, according to the teachings of the invention, for providing a notch in the edge of an electrically conductive sheet material;

FIGS. 4A and 4B illustrate the method shown in FIGS. 3A and 3B, for providing a notch in the opposite edge of an electrically conductive sheet material;

FIG. 5 illustrates the transposition of the sheet conductors shown in FIGS. 38 and 48;

FIG. 6 illustrates a sheet of electrical insulating material, which may be used to electrically insulate the two transposed sheet conductors shown in FIG. 5; and

FIG. 7 illustrates the sheet materials which formed the transposition of FIG. 5, electrically insulated from one another with the sheet of insulating material shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown an electrical winding of the type which may utilize the teachings of the invention. While winding 10 is shown as being rectangular, it may be round, or any other desired shape. Electrical winding 10, which may be a primary or a secondary winding for an electrical transformer, or a coil for a reactor or autotransformer, is constructed of at least two sheets 12 and 14 of electrically conductive sheet material, such as copper or aluminum. Sheets 12 and 14 may be strip or foil of any desired thickness T and width W, as required by the specific application. Typical thickness dimensions are in the range of 0.008 inch to 0.063 inch, and typical width dimensions are in the range of 10 inches to 56 inches, for electrical power transformers, but it will be understood that the invention will apply to the transposition of sheet conductors having any desired dimensions. For example, distribution type transformers may utilize sheet conductors having smaller thicknesses and width dimensions.

As the thickness dimension T of the sheet material increases, it becomes more difficult to wind the sheet material about a mandrel with a good space factor. Further, as the thickness dimension T increases, the losses due to eddy currents increase. Therefore, for both mechanical and electrical reasons, when the current requirements dictate a thickness dimension which exceeds, in general, 0.063 inch for power transformers, the required conductor cross-sectional area may be subdivided into a plurality of sheet conductors which are wound simultaneously and connected in parallel at the ends of the winding.

As shown in FIG. 1, sheet conductors 12 and 14 are connected to a power terminal 16, such as by welding; and the two sheet conductors are wound about a common axis 18, forming a plurality of superposed or nested conductor turns 20. The sheets of electrically conductive material may be wound directly upon an expandable mandrel, or they may be wound about a winding tube 22, which is disposed on a mandrel.

In order to prevent the two sheet materials from contacting one another, except at the ends of the winding, and at any tap connection points, and from contacting the conductors of adjacent turns, the two sheet materials must be electrically insulated. The electrical insulation between conductor turns, and between the multiple conductors of each turn, may be an insulating coating disposed on one or both of the major opposed surfaces of the sheet materials, such as an enamel coating formed of an epoxy, or other good electrical insulating material; or, the electrical insulation may be one or more sheets or strips of insulating material, such as kraft paper, which are disposed between the electrically conductive sheet materials, and adjacent the outer surface of at least one of the sheets, and wound simultaneously therewith.

After winding 10 is complete, the two sheet materials 12 and 14 will have their ends connected to another power lead, similar to the lead 16.

While dividing the conductive cross-sectional area required into a plurality of parallel connected conductive elements reduces the eddy current losses of the electrical winding, the reduction in losses may be offset by an increase in losses due to circulating currents in the parallel connected elements. Losses due to circulating currents may be reduced by transposing the relative radial positions of the parallel connected elements in attempt to subject all of the parallel connected elements to the same net leakage flux. With two parallel connected elements, such as shown in FIG. 1, a standard transposition wherein the radial sequence of the conductors is reversed, will produce a perfect transposition, if performed at the midpoint of the total conductor length, since each conductor occupies the relative radial position of the other for substantially the same conductor length. The standard transposition is shown schematically in FIG. 2A, wherein two conductors A and B are connected in parallel between terminals 25 and 27, and transposed at transposition point 24 to reverse their radial positions.

The sheet conductors l2 and 14, as shown in FIG. 1, start at terminal 16 with sheet conductor 12 being the inner conductor, and their relative radial positions are transposed or reversed at transposition 26, such that sheet conductor 14 becomes the inner conductor. As will be hereinafter described, the standard transposition 26 may be performed according to the teachings of this invention.

FIG. 2B is a schematic diagram illustrating how three sheet conductors A, B and C, connected in parallel, between terminals 28 and 30, may be transposed, using all standard transpositions, to provide a perfect transposition wherein each sheet conductor successively occupies the positions of the other sheet conductors, for substantially the same distance. The standard transposition alternates between the two outer conductors and the two inner conductors, until completing five transpositions. Thus, the two inner conductors B and C are transposed at point 32, the two outer conductors A and C are transposed at point 34, the two inner conductors A and B are transposed at point 36, the two outer conductors B and C are transposed at point 38, and the two inner conductors C and A are transposed at point 40.

In addition to making the standard transposition with two conductors, any number of conductors may be divided into two groups and the two groups transposed with a standard transposition. This type of transposition is shown schematically in FIGS. 2C, with conductors A, B, C and D being connected between terminals 42 and 44 and transposed at transposition 46. Transposition 46, however, since it does not reverse the positions of the conductors, is technically not a standard transposition, and is often called a complete transposition. Also, it is not a perfect transposition, as the conductors do not occupy the positions of all of the other conductors.

Four conductors may be perfectly transposed using a combination of standard and complete transpositions. This arrangement is shown schematically in FIG. 2D, with conductors A, B, C and D being connected in parallel between terminals 47 and 48. The first transposition point includes a complete transposition 50, the next point includes two standard transpositions 52 and 54, and the next transposition point includes a complete transposition 56. This arrangement causes each conductor to occupy each of the radial positions for the same distance. This result can also be achieved by using two standard transpositions at the first transposition point, a complete transposition at the next transposition point, and two standard transpositions at the last transposition point.

The different transposing arrangements shown in FIGS. 2A, 2B, 2C and 2D are examples of interleaving arrangements which may be used with electrically conductive sheet materials, according to the teachings of the invention. Other arrangements, however, may also be used.

Copending application Ser. No. 742,722, filed July 5, I968, now US. Pat. No. 3,546,644 which is assigned to the same assignee as the present application, discloses transposing arrangements for sheet type electrically conductive materials wherein a notch is disposed in opposite edges of the superposed conductors to be transposed, with each conductor proceeding to its transposed position through the notch in the other conductor. Also disclosed in this copending application are methods of forming a notch in electrically conductive sheet material, which relate to forming the notch while minifying the amount of scrap, minifying current crowding in the vicinity of the notch, and preventing the current density in the narrowed portion of the sheet from increasing substantially above that in the unnarrowed portion. The present application relates to new and improved methods of forming a notch in electrically conductive sheet materials, and also to new and improved methods of transposing sheet materials which do not require the relative positions of the rolls of supply material be changed after each transposition point.

FIGS. 3A, 3B, 4A, 4B, 5, 6 and 7 illustrate the steps ofa new method of forming notches in electrically conductive sheet material, which is essentially scrapless, and which maintains substantially the same current density in the portion of the sheet material adjacent the notch, as that in the unnarrowed portion. FIGS. 3A and 3B illustrate the steps of a method of forming a notch or opening, on one edge of an electrically conductive sheet material, with FIG. 3A illustrating a sheet 60 of electrically conductive material, such as copper or aluminum, having first and second edges 62 and 64, respectively, and first and second ends 66 and 68, respectively. End 66 may be from a partially wound electrical coil or winding, such as from winding shown in FIG. 1, which is wound to the point where transposition 26 is to be performed. End 68 is attached to a roll of supply material, disposed on a suitable payoff reel (not shown).

The first step is to cut the sheet 60 with first and second cuts 70 and 72, respectively, starting from the edge in which the notch is to be provided. In this instance, the sheet 60 is cut from the second edge 64. The cuts 70 and 72 are cut at predetermined angles 74 and 76 with respect to the edge, with the cuts converging towards one another, until reaching predetermined points with respect to the width of the sheet 60. They should proceed at least to the center line 78 of the sheet width, and preferably a short distance beyond, in order to simplify the electrical insulating of the transposition. The two cuts are spaced from one another such that they meet the predetermined point relative to the width of the strip a predetermined distance apart. The predetermined angles 74 and 76 shown in FIG. 3A are 30, as this has been found to provide excellent results. The exact angle, however, is not critical. If the angles 74 and 76 are too great, the transition zone between the unnarrowed width and the narrowed width of the sheet will be too short, causing undue current crowding, which in turn increases the temperature of the conductor in this area. If the angles 74 and 76 are too small, the notch and therefore the resulting transposition will become large with respect to the diameter of the conductor turns at the transposition point, which should be avoided. The angle of 30 has been found to be suitable, since the current density in the narrowed portion will be substantially the same as in the unnarrowed portion of the sheet, when following the teachings of the disclosed method.

The next step, shown in FIG. 3B, is to fold the section between cuts 70 and 72 along fold line 80, back against the portion of the sheet which is disposed between the ends of the cuts 70 and 72 and the edge 62, with the folding of this section automatically forming a notch or opening 82 in edge 64. If the cuts have extended past center line 78, the outer edge of the folded section will extend past edge 62 by a predetermined small amount, and this small excess may be trimmed to coincide with edge 62, if desired. The amount trimmed, however, is negligible compared with its affect on the current density, and it is the only scrap in this method.

The next step, also shown in FIG. 3B, is to electrically join the cut edges of the folded section to the immediately adjacent portion of the sheet conductive material, as shown at 84 and 86. The electrical joining may be accomplished by welding, soldering, brazing, or any other suitable method. The portion of the sheet material between the apex of the notch 82 and edge 62 is thus a double section, doubling the cross-sectional area of the conductive material in this area, which maintains the current density through the narrowed section of the sheet substantially the same magnitude as it is in the remaining portions of the sheet. If the folded sections were not electrically joined to the main portion of the sheet, the result would be the same as if the folded sections were removed from the sheets, which would then double the current density adjacent the notch and substantially increase the temperature of the strip in this area.

FIGS. 4A and 4B illustrate the method steps of FIGS. 3A and 3B, for providing a notch in the other edge of sheet conductive material. Specifically, FIG. 4A illustrates a sheet 90 of electrically conductive material having first and second edges 92 and 94, respectively, and first and second ends 96 and 98. End 96 may be from a partially wound electrical coil, and end 98 may be connected to a roll of supply material. Sheet material 90 is also provided with first and second cuts 100 and 102 which are disposed at angles 104 and 106 with edge 92. The angles 104 and 106 should be the same as angles 74 and 76 shown in FIG. 3A, and the cuts 100 and 102 should proceed to the same point relative to the strip width that the cuts 70 and 72 proceeded to in FIG. 3A. Thus, the cuts 100 and 102 should proceed just beyond the center line 108 of the strip or sheet conductor 90, with the section of conductor between the two cuts being folded along fold line 110, as shown in FIG. 4B. The cut edges of the folded section are then electrically joined to the immediately adjacent sheet material. as shown as 114 and 116. The folding of the sheet material between the cuts automatically forms an opening or notch 112 in the edge 92 of the sheet material.

FIG. 5 illustrates a transposition 120 formed with the prepared sheet conductors 60 and 90 shown in FIGS. 38 and 48, with sheet 60 proceeding to its new position through the notch 112 in sheet material 90, and sheet material 90 proceeding to its new position through the notch 82 in the sheet conductive material 60.

Since sheet conductors 60 and 90 have the edges of their folded sections electrically joined to the immediately adjacent surface of the sheet conductor, the plurality of parallel connected sheet conductors may be best insulated with a separate sheet of electrical insulating material, instead of an insulating coating applied to the sheet materials, which would have to be removed, at least in the area of the joint, for most types of joining methods. Thus, the electrical winding would have a sheet of insulating material disposed between each adjacent pair of sheet conductors, which would be wound simultaneously with the winding of the sheet conductors. The sheet insulating material which separates the two sheet conductors to be transposed, may be prepared as shown in FIG. 6, at the point which is to be adjacent the transposition of the sheet conductors. FIG. 6 illustrates a sheet 124 of insulating material having first and second edges 126 and 128, and having cuts 130 and 132 which start at edges 126 and 128, respectively. Cuts 130 and 132 may be made along lines which are perpendicular to the edges 126 and 128, or they may be cut at an angle thereto, such as at the same angle which the notch makes with the edge ofthe sheet-conductive material. Cut 130 should proceed towards edge 128 at least for the dimension necessary to receive the portion of sheet material 60 between edge 62 and fold line on sheet material 60, and cut 132 should proceed towards edge 126, at least for the dimension necessary to receive the portion of sheet material between edge 94 and fold line 110.

FIG. 7 illustrates the transposition shown in FIG. 5, with sheet-insulating material 124 shown in FIG. 6 disposed between sheet conductors 60 and 90, on both sides of the transposition point. Sheet conductive material 60 proceeds through the opening provided by cut 130 in the sheet of insulating material 124, and sheet-conductive material 90 proceeds through the opening provided by cut 132,

The method of forming the transposition 120 shown in FIG. 5 provides a transposition having negligible scrap, without requiring that any of the conductors be turned over as they proceed through the transposition point, without completely severing a conductor, and without substantially increasing the current density through the narrow portion of the conductor adjacent the notch. However, transposition 120 does require that the positions of the two reels of conductive material which supply sheet conductors 60 and 90, have their relative positions reversed after the transposition point. This could be accomplishes by exchanging the positions of the supply rolls on the payoff reels, or by moving the payoff reels and their associated supply rolls.

In summary, there has been disclosed new and improved methods of forming transpositions of sheet conductive materials, and new and improved methods of forming notches in the edges of the sheet conductive materials, preparatory to the making of a transposition. The sheet conductors to be transposed are not severed completely, the methods are essentially scrapless, and they perform the transposition without substantially increasing the current density adjacent the notch.

We claim as our invention: 1. A method of transposing superposed electrically conductive-sheet materials, comprising the steps of:

cutting a first sheet material, having first and second edges, from the first edge thereof, at first and second spaced points, to form first and second cuts having predetermined angles relative to the first edge, the first and second cuts proceeding at least to the midpoint of the sheet width,

folding the section of sheet material disposed between the first and second cuts against the portion of the sheet material located between the ends of the first and second cuts and the second edge of the sheet material, to form a notch in the first edge of the first sheet material,

electrically joining the edges of the folded section to the portion of the sheet material it is folded against,

cutting a second sheet material having first and second edges, from the second edge thereof, at first and second spaced points to form first and second cuts having predetermined angles relative to the second edge, with the first and second cuts proceeding at least to the midpoint of the sheet width,

folding the section of sheet material disposed between the first and second cuts against the portion of the sheet material located between the ends of the first and second cuts and the first edge of the sheet material, to form a notch in the second edge of the second sheet material, electrically joining the edges of the folded section to the portion of the sheet material it is folded against,

and transposing the relative positions of the superposed first and second sheet materials by directing the first sheet material through the notch in the second sheet material, and by directing the second sheet material through the notch in the first sheet material.

2. The method of claim 1 including the step of providing a sheet of electrical insulating material having first and second edges, cutting the sheet of electrical insulating material to provide first and second cuts which extend inwardly from the first and second edges thereof, respectively, for a predetermined dimension, disposing the sheet of electrical insulating material between the first and second sheets of electrically conductive material, with the first sheet of electrical conductive material proceeding through the second cut in the sheet of insulating material, and the second sheet of electrical conductive material proceeding through the first cut, at the point of transposing the first and second electrically conductive sheet materials.

3. The method of claim 1 including the step of providing first and second sheets of electrical insulating material each having first and second edges, cutting each of the first and second sheets of electrical insulating materials inwardly from one edge thereof, and interleaving the first and second sheets of electrical insulating material with the transposed first and second sheets of electrically conductive material, with the first electrically conductive sheet material and the first sheet of insulating material proceeding through the cut in the second sheet of insulating material, and with the second electrically conductive sheet material and the second sheet of insulating material proceeding through the cut in the first sheet of insulating material. 

1. A method of transposing superposed electrically conductivesheet materials, comprising the steps of: cutting a first sheet material, having first and second edges, from the first edge thereof, at first and second spaced points, to form first and second cuts having predetermined angles relative to the first edge, the first and second cuts proceeding at least to the midpoint of the sheet width, folding the section of sheet material disposed between the first and second cuts against the portion of the sheet material located between the ends of the first and second cuts and the second edge of the sheet material, to form a notch in the first edge of the first sheet material, electrically joining the edges of the folded section to the portion of the sheet material it is folded against, cutting a second sheet material having first and second edges, from the second edge thereof, at first and second spaced points to form first and second cuts having predetermined angles relative to the second edge, with the first and second cuts proceeding at least to the midpoint of the sheet width, folding the section of sheet material disposed between the first and second cuts against the portion of the sheet material located between the ends of the first and second cuts and the first edge of the sheet material, to form a notch in the second edge of the second sheet material, electrically joining the edges of the folded section to the portion of the sheet material it is folded against, and transposing the relative positions of the superposed first and second sheet materials by directing the first sheet material through the notch in the second sheet material, and by directing the second sheet material through the notch in the first sheet material.
 2. The method of claim 1 including the step of providing a sheet of electrical insulating material having first and second edges, cutting the sheet of electrical insulating material to provide first and second cuts which extend inwardly from the first and second edges thereof, respectively, for a predetermined dimension, disposing the sheet of electrical insulating material between the first and second sheets of electrically conductive material, with the first sheet of electrical conductive material proceeding through the second cut in the sheet of insulating material, and the second sheet of electrical conductive material proceeding through the first cut, at the point of transposing the first and second electrically conductive sheet materials.
 3. The method of claim 1 including the step of providing first and second sheets of electrical insulating material each having first and second edges, cutting each of the first and second sheets of electrical insulating materials inwardly from one edge thereof, and interleaving the first and second sheets of electrical insulating material with the transposed first and second sheets of electrically conductive material, with the first electrically conductive sheet material and the first sheet of insulating material proceeding through the cut in the second sheet of insulating material, and with the second electrically conductive sheet material and the second sheet of insulating material proceeding through the cut in the first sheet of insulating material. 